1. Field of the Invention
2. Discussion of Background
Hydrogen-absorbing materials are used in many applications, including processes involving the storage, recovery and supply of hydrogen. Industries such as hydrogen processing and energy conversion use such materials in hydrogen purification and separation processes.
Various metals and metal alloys can absorb and then desorb large amounts of hydrogen under appropriate temperature and pressure conditions. These materials are referred to as metal hydrides and are well known in the art. They include pure metals such as Mg, Pd, Ti, Pt, U, and alloys such as those based on nickel, lanthanum and aluminum.
Metal hydrides are used in many different forms. Although frequently used in the form of granules, metal hydrides are sometimes incorporated into a matrix such as a polymer. (See, for example, U.S. Pat. No. 4,110,425, issued to Buhl et al.)
Similarly, porous polymeric matrices are used as media for supporting metal hydrides and other hydrogen-absorbing materials because of the increased surface area and corresponding increase in the amount of hydrogen that can be absorbed in a given volume. Such compositions are disclosed in U.S. Pat. Nos. 4,433,063, issued to Bernstein et al, and 4,036,944, issued to Blytas.
However, current support media for hydrogen-absorbing materials are often insufficient to withstand the rigors of repeated hydrogen absorption/desorption cycles typical of industrial processes. Repeated cycles of absorption and desorption tend to break the hydride particles down into fine powders that compact within the matrix and block gas flow. Also, the powder particles will escape from the support matrix and clog gas lines, filters and other process components.
To eliminate these problems, continuous efforts are being made to develop alternate support media for hydrogen-absorbing materials. Such efforts have led to the production of porous glass materials, particularly porous glass matrices made by sol-gel processes.
Due to their high porosity, sol-gel glasses have very large specific surface areas, an important factor affecting the absorption capacity of hydrogen-absorbing material incorporated into such support matrices. Sol-gels are known as supports for reagents interacting with solutes or other components. Many of such uses are described, for example, in European patent 0 439 318 A2, issued to Avnir et al. However, until recently, sol-gels have never really been adapted for hydrogen-absorbing applications.
Our commonly-assigned and recently-filed U.S. patent application (Ser. No. 07/968,641, filed Oct. 10, 1992) discloses a hydrogen-absorbing composition prepared by a sol-gel process. In that application, a sol is prepared from an organometallic compound, such as tetraethoxysilane, and mixed with hydride particles. The mixture is allowed to polymerize and then to cure to form a highly porous matrix having hydride particles dispersed throughout.
There is a need for a hydrogen-absorbing composition that can perform repeated absorption/desorption cycles without having significant breakdown and consequent release of the hydride particles into the process stream.